High temperature solid oxide electrolyte fuel cells (SOFC) have demonstrated the potential for high efficiency and low pollution in power generation. Successful operation of SOFCs for power generation has been limited in the past to temperatures of around 1000° C., due to insufficient electrical conduction of the electrolyte and high air electrode polarization loss at lower temperatures. U.S. Pat. Nos. 4,490,111 and 5,916,700 (Isenberg and Ruka et al. respectively) disclose standard, solid oxide tubular type fuel cells, which could operate at the above described relatively high temperatures. They comprised a solid oxide electrolyte disposed between a fuel electrode (anode) and an air electrode, which was self-supporting or supported by a separate support structure. These layers were deposited by various techniques such as vapor halide deposition. Isenberg, et al., in U.S. Pat. No. 4,547,437 describes in some detail these vapor deposition methods and methods such as slurry spraying/sintering or plasma—flame—spraying to deposit, specifically, an interlayer material over the air electrode, next to the solid oxide electrolyte.
In addition to large-scale power generation, SOFCs which could operate at lower temperatures would be useful in additional applications such as in powering light-duty vehicles. These tubular cells however have a relatively low power density even at 1000° C. and have a potential to bow after curing.
Cells of a flattened parallel sided cross section, that have a number of ribs connecting the adjacent paralleled sides of a lanthanum manganite air electrode extrusion, have achieved substantially higher power density than the cylindrical cells, and are candidates to form the basic element of the next generation of SOFC generators, see FIG. 1 of the drawings. These cells are described in U.S. Pat. Nos. 4,874,678 and 4,888,254 (both Reichner) and U.S. Patent Application Publications U.S. 2007/0160886 A1 and U.S. 2007/0243445 A1 (both Digiuseppe). Air flows within discrete passages that are formed between the ribs and flat sides of the air electrode, preferably through air flow tubes.
These flattened cells are referred to as HPDX cells, where HPD indicates “high power density” and X indicates the number of air passages. In HPD cells a lanthanum chromite interconnection is preferably deposited over the entirety of one flat face of the air electrode. A yttria stabilized zirconia electrolyte covers the opposite face and the rounded edges of the air electrode so as to overlap the edges of the interconnection surface but leave most of this surface exposed. An idealized section view of a typical fuel electrode is shown in FIG. 2, from U.S. Pat. No. 4,582,766 (Isenberg et al.), showing large nickel or cobalt particles held in place by partially surrounding electrolyte material. While these metals are preferred, the patent does mention iron, copper and chromium particles. A standard nickel/yttria stabilized zirconia cermet fuel electrode covers the electrolyte except for a narrow margin of electrolyte that surrounds the interconnection. Series electrical connection between cells is accomplished by means of a nickel felt structure a flat face of which is sintered to the interconnection while the raised ribs of which are sintered to the fuel electrode face of the adjacent cell.
A major problem is the cost of nickel for the fuel electrode anode. Also, the fuel electrode anode is sulfur intolerant and can be readily poisoned by ppm level sulfur that is found in many fuel gases derived from coal. This problem was partly solved by Ruka et al. in U.S. Pat. No. 5,021,304, where layers of large nickel particles were covered with fine nickel or cobalt particles coated with cerium oxide or strontium titanate.
Yet another problem, pointed out by Nguyen Q. Minh in “Ceramic Fuel Cells”, J. Am. Ceramic Society, 76 [3] 563-588, 1993, is that hydrogen, the most common fuel for use in SOFC has a high electrochemical reactivity, providing the following reaction at the nickel anode:H2+O2−═H2O+2e−,hydrogen oxidation at the nickel electrode produced water and adsorption of hydrogen on nickel, followed by the electrochemical reaction:Raymond J. Gorte et al. in “Anodes For Direct Oxidation of Dry Hydrocarbon In A Solid Oxide Fuel Cell” Advanced Materials [2000], 12, No. 19, October 2, pages 1465-1469 (hereinafter “Gorte et al. article”) state that with few exceptions, hydrocarbon-fueled systems depend on a reforming reaction and that the catalytic properties that make nickel an excellent reforming catalyst also prevent it from being a good choice as an electronic conductor in a direct-oxidation SOFC. Ni is used commercially as a hydrocarbon reforming catalyst, where coking is a significant problem if high steam-to-hydrocarbon ratios, typically greater than two, are not maintained. In dry CH4, Ni forms carbon fibers above 700° C., a serious problem since these fibers can completely fill the anode compartment. Attempts have been made to modify the Ni properties, by mixing it with Mn for example. However, the high operating temperatures and the tendency of Ni to catalyze formation of very long graphite fibers in dry hydrocarbons make Ni anodes very susceptible and can cause failure.
The Gorte et al. article further states that the high temperatures needed in SOFC production for achieving a gas-tight electrolyte membrane, preclude the use of low-melting metals for directly substituting Ni. However, replacement of the metal with an electronically conductive metal oxide, having a higher melting point than the metal, is promising because most oxides exhibit low carbon-formation rates; however, the challenge with oxides is to develop materials that are electronically conductive enough to achieve reasonable performance.
The Gorte et al. article focused further research on Cu-based anodes, which is less expensive than Ni, and has a m.p. of 1083° C. vs. 1453° C. for Ni. Problems using Cu in a yttria stabilized zirconia (YSZ) base were noted, because densification of YSZ requires heating to 1300° C. and Cu2O melts at 1235° C. Methods tried by the Gorte et al. article include aqueous impregnation of Cu(NO3)2 into a porous YSZ layer followed by calcination, but the anodes produced were said to have poor mechanical strength. Another method added pore formers to one side of a YSZ tape followed by co-sintering to produce pores, then impregnation with a copper solution.
This base Gorte et al. article, published in 2000, appears to have spurred additional interest in Cu based SOFC anodes, for example: U.S. Pat. Nos. 6,589,680 B1; 6,939,637 B2; 6,811,904 B2; 6,844,099 B1; 7,014,942 B2; and 6,958,196 B2 (all Gorte et al.). The base patent of the series, U.S. Pat. No. 6,589,680 B1 utilizes a calcined, porous substrate of zircon fibers and YSZ into which a salt solution of either Cu or Ni is impregnated.
Other articles based on this initial Gorte et al. article, for example, Park et. al, in “Direct Oxidation of Hydrocarbon In A Solid Oxide Fuel Cell” Nature, 404, 265-267, March 2000, dealt with the power density values of SOFCs having anodes of 40% wt. % Cu and 20 wt % CeO2 held in place by a YSZ matrix, operating at 700° C. (973K): C. Lu et al. “SOFCs For Direct Oxidation Of Hydrocarbon Fuels with Samaria—Doped Ceria Electrolyte” J. Electrochemical Soc. 150 (3) A354-A358 (2003) (hereinafter “Lu:JES Article”) a taught Sm0.2Ce0.8O1.9 (SDC) electrolyte layer that can operate at 600° C. with hydrogen as fuel, where a porous top SDC layer was used as anode and impregnated with Cu or mixtures of Cu and CeO2 from Cu or Ce nitrate solutions. These layers were made by uniaxially pressing a bilayer of pure SDC powder and SDC powder containing pore formers.
C. Lu et al. in “Copper Sintering In Cu-Ceria—Sm0.2Ce0.8O1.9” Mid Pacific Conference Center Meeting Symposium on Electrode Materials. University of Pennsylvania. Oct. 6, 2004, described the power density of Cu-Ceria based SDC anodes, having about 25 wt % Cu, at 700° C. (973K), using a pressing technique of fabrication described in Lu:JES Article, and also a tape casting method. Also, to suppress Cu sintering in the anode tantalum oxide is included in the anode structure but Cu sintering still remained a problem. Also S. Lee in “Cu—Co Bimetallic Anodes for Direct Utilization of Methane in SOFCs” Electrochem Solid-State Letters, 8 (1) A48-A51 (2005) attempts to solve Cu anode sintering problems over 800° C. (1073K) and taught Cu—Ni alloy anodes superior to Ni cermets but that a Cu—Co bimetallic system was even more impressive in countering carbon build up when exposed to n-butane in a SOFC operating at 700° C. (973K). Tape casting techniques were used to fabricate the anode.
In the area of planar fuel cells as differentiated from HPDX fuel cells, contact layers disposed between a conducting current take off plate and at least one of the electrodes have been discussed in U.S. Pat. Nos. 6,074,772 and 6,620,541 B2 (Hofer et al. and Fleck et al.). Other types of solid oxide fuel cells, can operate at higher current density than cylindrical cells, with improved packing density; the so called triangular or Delta X cells. However, they share the same anode problems. These triangular cells would also benefit from lower cost anodes and lower operating temperatures. Some examples of these triangular type solid oxide fuel cells include U.S. Pat. Nos. 4,476,198; 4,874,678 (FIG. 4); and U.S. Patent Application Publication U.S. 2008/0003478 A1 (Ackerman et al.; Reichner, and Greiner et al. respectively.)
As is obvious from the previous background, under today's energy conservation efforts, there is a need for lower operating temperature solid oxide fuel cells which could include improved, less expensive anodes not subject to sulfur or carbon fouling or metallic sintering. These sulfur and carbon problems date back at least fifteen years. Therefore, it is one of the main objects of this invention to provide new anode systems and new methods of producing those anodes. It is another of the main objects of this invention to provide lower cost anodes, operable at lower temperatures and resistant to metal sintering.